Hox proteins and their transcriptional co-factors have crucial roles in specifying motor neuron identity but their role in directing the assembly of sensory-motor circuits remains less clear. This proposal takes advantage of the discovery that the forkhead transcription factor FoxP1 gates the output of the entire motor neuron Hox repertoire, and determines the subtype identity of spinal motor neurons. We will use motor neuron-specific FoxP1 mutant mice to probe three aspects of the way in which motor neuron pool specification nucleates the organization and function of this prototypic mammalian sensory- motor circuit. First, we will explore the role of Hox/FoxP1 programming in the formation and maintenance of topographic motor projections to target limb muscles. Anatomical tracing methods will be used to define the intraspinal position of lumbar motor neurons that innervate specific limb muscle targets in the absence of FoxP1 function, asking whether motor neuron cell bodies that innervate a given muscle target are positioned randomly within the limb-innervating cohort. In addition, we will use physiological recording and kinematic analyses to assess muscle activity patterns and locomotor behavior in mice in which motor neuron Hox/FoxP1 programming has been abolished. Second, we will examine how the loss of motor pool identity influences patterns of proprioceptive sensory connectivity. Anatomical tracing and physiological recordings from spinal motor neurons will compare the profile of monosynaptic connections between antagonist pairs of sensory and motor neurons in control and motor neuron-specific FoxP1 mutant mice. In parallel, we will determine the impact of loss of motor pool identity on the molecular specification of subsets of proprioceptive sensory neuron that supply individual limb muscles. Third, we will examine how the loss of motor pool identity in FoxP1 mutants perturbs the selectivity of assembly of local inhibitory circuits. Physiological studies will determine how the pattern of connectivity of Ia inhibitory interneurons with motor neurons is altered in motor neuron specific FoxP1 mutants. Molecular markers will be used to identify Ia inhibitory interneurons and examine how the erosion of motor pool identity influences the output and connectivity of this set of interneurons. Together, these studies are intended to forge a link between molecular programs of motor circuit assembly and the behavioral output of these circuits. In the long term, these insights should aid in the design of more effective strategies for promoting functional recovery of spinal circuits after traumatic spinal cord injury, and in motor neuron degenerative diseases. PUBLIC HEALTH RELEVANCE: The activity of spinal motor neuron circuits is critical for movement, and damage to these circuits - through traumatic injury or in neurodegenerative disease - is a major fiscal and societal burden. The research outlined in this proposal aims to define the cellular and molecular mechanisms that control the organization of spinal motor circuits, opening the way for more rational approaches to the treatment of spinal cord injury and motor neuron diseases.